Range headers and native HTML5 streaming support in Backrunner

HLS protocol is a bit different, but yet again player uses Range header to specify position within timeframe.

Previously Elliptics HTTP server Backrunner used size/offset URI parameters for this purpose, which are not ajax friendly and obviously are not supported by standard players.

With this new Backrunner update we add Range and If-Modified-Since headers support.
The former allows to work HTML5 pleers with Elliptics HTTP proxy out of the box. If-Modified-Since is quite useful for client-side caching.